A conventional optical communication module is adapted to absorb position discrepancies between a printed circuit board and the substrate of an optical module that is for transmission or reception, by electrically connecting electrodes of the substrate and electrodes of the printed circuit board through a flexible circuit board (see, e.g., Japanese Laid-Open Patent Publication No. 2003-249711). Meanwhile, another optical communication module is adapted to prevent drops in transmission characteristics consequent to variation of the electrical property of the flexible circuit board caused by the influence of the printed circuit board, by electrically connecting electrodes of the printed circuit board to electrodes of the flexible circuit board through another substrate for relaying therebetween (see, e.g., Japanese Laid-Open Patent Publication No. 2008-187670).
When a flexible circuit board connected to electrodes of an optical modulating device is electrically connected to a driver on a printed circuit board through signal wiring of the printed circuit board, the modulation band becomes narrow because propagation loss is high for a high-frequency signal propagated in the signal wiring of the printed circuit board. The propagation loss results in reduced amplitude of the signal input into the optical modulating device and thus, a higher output driver is necessary to compensate for the reduced amplitude. Consequently, power consumption increases. Mismatching of impedance occurs in each of the soldered portions between the electrodes of the printed circuit board and the electrodes of the flexible circuit board, and the soldered portions between the electrodes of the printed circuit board and lead pins of the driver. Thus, an incident wave reflection coefficient S11 is degraded.
The electrodes of the flexible circuit board connected to the electrodes of the optical modulating device may be directly soldered to the lead pins of the driver rather than through a printed circuit board. By doing this, propagation loss that occurs when a high-frequency signal propagates in the signal wiring of the printed circuit board is eliminated, whereby the narrowing of the modulation band can be suppressed. The reduction of the amplitude of the signal input into the optical modulating device can be suppressed and thus, increases in power consumption can be suppressed. Further, the reduction of one soldered portion improves the incident wave reflection coefficient S11.
Since the optical modulating device cannot withstand high temperatures, a soldering process employing reflow is not applicable to the soldering of the electrodes of the flexible circuit board connected to the electrodes of the optical modulating device, to the lead pins of the driver. Therefore, using a soldering iron, a worker has to manually solder to the lead pins of the driver mounted on the surface of the printed circuit board, the electrodes of the flexible circuit board connected to the electrodes of the optical modulating device.
However, the interval between the driver and the optical modulating device is small because a shorter distance for a high-frequency signal to propagate can suppress greater propagation loss. Therefore, a problem arises in that the worker is unable to insert the tip of the soldering iron between the driver and the optical modulating device when soldering the electrodes of the flexible circuit board connected to the electrodes of the optical modulating device to the lead pins of the driver.